Signal transmission system



sumo s'r 'rEs PATENT OFFICE assasss v r I SIGNAL raamsmssron srs'rmGeorge W. Gilman, Short'llills,.N. 1., aaeignor to Bell TelephoneLaboratories, Incorporated, New York, N. 1., a corporation of New YorkApplication March re, 1942, Serial No. 435,235 3 Claims. (01. 178-44)This invention relates to electrical transmission systems and moreparticularly to electrical filter systems. Still more particularly thisin- I vention relates to transmission systems employin: filters'orsimilar circuits or networks set up to transmit currents having a bandof frequencies, the width of which may be changed in accordance withsignal currents or voltages or other causes.

This invention involves a band selective network having limits which arevariable under control of a signal current or voltage or other effect.The network may be supplied, for example, with a band of frequenciescomprising a substantially continuous spectrum of randomly relatedcurrents extending over a wide band, such as the band that may beobtained from a noise generator or other source, and this band may bewidened v or narrowed according to signals or other causes.

The network of this invention may include, for example, an oscillatorfor su plying current having a frequency which directly depends on thesignal voltage or current, and a modulator for superimposing thereceived current on the oscillator current for raising the received bandof frequencies in the frequency spectrum. The currents of the elevatedfrequencies may then be transmitted through a band filter of fixedlimits which may transmit only a portion of the elevated frequencies.The frequencies transmitted by the filter may then be demodulated withthe same oscillator current to return theelevated frequenciestransmitted by the filter to their original position in the frequencyspectrum. The demodulated band of frequencies will be of a width whichwill correspond to the magnitude of the impressed signal current orvoltages or other cause. The filters employed may be of any suitabletype,

' such as the crystal filters disclosed in Patent 2,045,991 granted June30, 1936. to W. P. Mason. This invention will be better understood fromthe more detailed description hereinafter following when readinconnection with the accompanyingzdrawing in which the circuitarrangement shown illustrates one form of apparatus for producing a bandof frequencies of a width which depends on a signal current or voltageor other factor.

. Referring to thedrawing. there is shown an input circuit L; which mayreceive from a source 50 cycles according to the magnitude of the con--electrons in a resistive impedance or the random fluctuations of theelectron current in the space path of a thermionic vacuum tube. Theinput circuit Ll is connected to a band filter F1 which may transmitonly a predetermined band of frequencies as, for instance. 1000 to 1100cycles; all other received frequencies being suppressed by filter F1.The transmitted currents together with the current of an oscillator Oare fed to a balanced modulator MD which .is of well-known type andserves to produce two side bands of the current obtainedrfrom oscillator0, the current of oscillator 0 being suppressed in the balanced circuitsof the modulator. The oscillator O is of the variable-frequency type,the frequency of which is varied in accordance with variations in theamplitude or phase of the signal current or voltage or other effectreceived over a signal control circuit In. The oscillator frequency mayhave a mean value of, for example, 148,950 cycles and many swing over a.range of plus or minus trol current or voltage. I

Assuming that the signalcurrent or voltage in circuit La is of zeromagnitude or is entirely abmean value of 148,950 cycles. and the lowerside band resulting from modulation will extend from 147,850-147,950cycles while the upper side band will extend from 149,950-150,050cycles. These side bands are then transmitted to a second band filter B:which has fixed limits of, for example, -156 kilocycles. This filterwill suppress all of the lower side band obtained from modulator *MD aswell as part of the upper side band. i. e. the part below the lowercutofl frequency of the filter. Hence those frequencies of the upperside band which are above the lower cutoil' point of the filter F: willalone be transmitted, or i50,000150,050 cycles. A band of this widthtraversing filter F2 and evelated in the frequency spectrum willcorrespond to an oscillator frequency which is at its mean value due tothe absence of a control signal.

Currents of the frequencies passed by fllter Faare then fed along withthe oscillator current to a balanced demodulator DM of well-known type,the demodulator acting to return the elevated band of frequencies to itsoriginal position in the frequency spectrum. The band of frequenciesobtained from the demodulator DM in the illustration given will extendfrom 1050-1100'cycles and currents of these frequencies will appear inthe output circuit La leading to a load circuit The reduced band offrequencies returned to its the band. An example of this lng action willnow be given.

original position in the frequency spectrum will correspond to thecondition of zero control voltage. The demodulator DM also creates anupper side band which may be eliminated by any simple selective means(not shown) such as an output transformer of suitable characteristic ora lowpass filter.

Instead of a control voltage. in line Ls of zero value, the controlvoltage may be positive or negative within certain limits. when thecontrol voltage increases and reaches its maximum positive value, thefrequency of the oscillator current will rise to. for example, 149kilocycles. when this happens, the upper side bandresulting frommodulation in modulator MD will extend from 150,000 to 150,100 cycles,all or the components oi which will lie within the frequency limits offilter F2. All of the-upper side band will therefore be transmitted byfilter F: and the entire lower side band will be' suppressed. This upperside band will be returned by the action of demodulator DM to itsoriginal position in the frequency. spectrum, the demodulated bandincludme filter Fr. There will then. be substantially an absenceoicurrent in line Ls.

Thus it will be observed that with an oscillator supplrins a currentwhich varies in frequency in accordalice' with signals or'other effects,the band of frequencies fedto the output circuit Lswill be changed inwidthto correspond to the values oi the signals, or other controleiiects. The band may be increased in width to limits which aredetermined by the frequency limits of filters F1 and 1'5, whichever isthe narrower.

It has been shown that when the oscillator current frequen'cyis belowthe lower frequency limit of filter'l 'z, only the upper side bandresulting from modulation contains components within the range-oi filter1": which are transmitted and traversing filter 1'1.

thereafter demodulated. It is the lower regionot the upper side bandthat may be trimmed" or cut off due to the presence or absence of acontrol voltage as already explained. In a case where the oscillatorcurrent frequency is above theup-' per limit of filter 1'5 the upperregion of the lower side band resulting from modulation .will

likewise be "trimmed" or removed due to the presence or absence of acontrol voltage.

filter'l 'i while not ,afiectingthe'lower-edge of type of modulat-Assume, for example, that the irequency' limits of filter I"; are10,000-l0,100 cycles, that thsmean frequency of oscillator O'is145,950cyeies and that the oscillator within limits of plus or minus cycles inacascaacs side band will extend from 155,950 to 156,050 cycles. If thefilter I": has the frequency limits of 150-156 kilocycles, as was alsoassumed in prior illustrations, then the lower side band will beentirely suppressed while only part of the upper Fr, 1. e. 10,050-10,100cycles, will be suppressed or trimmed. The demodulator DM- also createsan upper side band which may be eliminated by any simple selective means(-not shown) such as an output transformer of suitable characteristic ora lowpass filter.

As the frequency of oscillator O rises to, for

example, itsmaximum value of 146 kilocycles, then the upper side bandresulting from modulation will extend from 156,000 to 156,100 cycles.All of this side band (and of course the lower side band) will beoutside the limits of filter I": and will therefore be suppressed. Theline 14 will therefore receive no current during this status. v

However, when the oscillator fmquency falls to. for example, its minimumvalue, or 145,900 cycles, the upper side band will comprise thefrequencies between155,900 and 156,000 cycles, all of the components.of-which come within the range filter 1'5. Upon demodulation the bandwill comprise irequenciee between 10,000 and 10,100 cycleswhichhasthesamewidthastheoriginalband Itwill be noted that the uppere'dge'oi the band traversing filter 1"1 may be "trimmed" by having thefrequency spacing between the side bands resulting from modulationgreater than thegband width of filter 1h. The filter 15 must have a passband which iswide enough to permitoneoi'thesidebandstobesweptaemssitseue ataratetotheirequencyofthemodulating sisnal voltage or current incircuitlisl Whether-the upper orloweredge oi the band is trimmed is determined by the side bandwhichcomeswithinthersngeoifilterhand bythe' edge of the latter filterwhich the side band is swept across. This may be seneralissd as follows:When the lower side band from modulatorlmsweepsacrosstheloweredsoofthefilterhtheupperportionotthebandtraveru ing filter l'i will'have been"trimm Whenhave been trimmed, it this same side'band.

frequency is'variabl'e- I corda'nce with any ana] orcontrol voltage-inoircuitla as assumed in previous exsmplis. At (I is may beampllfisdifdesired, and modulated casesthe"trimmingozthebandoffilterl ris ,of afrequency width which depends on and corresponds-to the signal currentor voltage in circuith. w

Itwillbeunderstoodthatthecurrentsinline distant point the received bandof frequencies may be amplified, if desired, demodulated and thereafterimpressed upon a square law detector of well-known type to reproduce theoriginal speech or other signals corresponding to the speech or othersignals in circuit La. The currents of line Ls may be transmitted bysystems of amplitude modulation or frequency modulation as will be clearto those skilled in the art.

It will be apparent that by arranging two networks, such as that shownin the'drawing, both connected in tandem, the trimming action may becaused to take place at both ends of theband simultaneously and, ifdesired, equal changes in width at the two edges of the band may beobtained.

The structure used to vary the frequency of the current or oscillator inaccordance with the signal waves or other effects received over line Lamaybe any suitable well-known type, such as the structure" disclosed inPatent 2,250,104 granted be of any desired width.

The frequencies used in the examples and the swings in the oscillatorfrequency are given merely for illustrative purposes and are not to beconstrued as limitations on the invention.

'- While this invention has been shown and described. in certainparticular embodiments merely.

for the purpose of illustration, it will be understood that the generalprinciples of this invention may be applied to other and widely variedorganizations without departing from the spirit of the invention and thescope of the appended claims.

What is claimed is:

1. A signal transmission system ting a frequency band having its widthvaried in accordance with signals, comprising a source of alternatingcurrent, a source of signals, means for varying the frequency of saidalternating curfor .transmitrent in accordance with said signals, asource supplying a substantially continuous band of frequencies having apredetermined constant width, and means responsive to changes in thefrequency of said alternating current to vary proportionally theinstantaneous width of the band of frequencies.

2. The combination of a source of currents having energy substantiallycontinuously distributed over a band of frequencies of predeterminedlimits, said limits being fixed and constant, an, oscillator, anindependent source of variable voltage, means for varying the frequencyof said oscillator in accordance with changes in the voltage of thelatter source, and means for changing the instantaneous width of theband of frequencies transmitted from the first source in proportion tothe varying voltage, said means including a modulator and a filter and ademodulator all connected in tandem, the oscillator being connected incommon to the modulator and the demodulator.

3. A' band selective transmission system, said system includin a sourceof signal currents havvary. in accordance with variations in theinstantaneo'us amplitude of the signal currents, a second band passfilter having a pass band of substantially greater width than said bandof frequencies and adapted to eliminate one of said side-bands and totrim part of the other sideband, the amount trimmed being proportionalto the instantaneous amplitude of said signal currents, and a secondmodulator for restoring to their original positions in the frequency.spectrum those frequencies from said generator that were passed by thesecond band pass filter.

GEORGE w. GIIMAN.

